1. Field of the Invention
The present invention relates to the measurement of certain physical properties of fluids and, more particularly, to the determination of the thermal conductivity, pressure and/or temperature of fluids.
2. Description of the Prior Art
A number of approaches have been devised to measure the thermal conductivity and other properties of a fluid of interest. One such approach is described in U.S. Pat. No. 4,735,082 in which thermal conductivity is detected using a Wheatstone bridge technique in which a filament in one leg of the bridge is placed or positioned in a cavity through which the sample gas of interest is passed. The filament is used to introduce a series of amounts of thermal energy into the fluid of interest at various levels by varying the input voltage which, are, in turn, detected at another leg as voltage difference signals. Integration of the changes of the value of the successive stream of signals yields a signal indicative of the heat dissipation through the fluid, and thus, the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, especially with reference to prior art FIGS. 1-5, very small and very accurate "microbridge" semiconductor chip sensors have been described in which etched semiconductor "microbridges" are used as heaters and sensors. Such sensors might include, for example, a pair of thin film sensors around a thin film heater for measuring flow rates. Semiconductor chip sensors of the class described are treated in a more detailed manner in one or more of patents such as U.S. Pat. No. 4,478,076, U.S. Pat. No. 4,478,077, U.S. Pat. No. 4,501,144, U.S. Pat. No. 4,651,564, and U.S. Pat. No. 4,683,159, all of common assignee with the present invention.
One interesting approach to measuring the thermal conductivity, k, of a fluid using a microbridge structure is disclosed in U.S. Pat. No. 4,944,035 to Aagard et al. Aagard et al. discloses using a heater film and at least one spaced sensor films to measure the thermal conductivity, k, of the fluid of interest. The heater film is energized for a relatively long period of time so that the temperature of the fluid, and thus the spaced sensor or sensors, reach and maintain a relatively constant value. During this time, one or more Wheatstone bridge structures incorporating the sensor or sensors provides an output signal that represents the voltage imbalance caused by the temperature change in the microbridge sensor or sensors. The amplitude of this imbalance is related to the thermal conductivity, k, of the fluid as shown specifically in FIG. 13 of Aagard et al. Using previously derived calibration data, the thermal conductivity can thus determined.
A limitation of this approach is that both a heater element and at least one sensor element are required to measure the thermal conductivity of a fluid. Another limitation is that relatively long heater pulses are required to allow the temperature of the spaced sensor element to reach and maintain a constant value. Thus, it would be desirable to provide a sensor that can determine the thermal conductivity, k, and other fluid properties, in a relatively short period of time using only one element such as a heater element.